Hmm. But there was that whole bit at the very end where John was like, "wait, if you only poisoned the sugar, why was I drugged?" and Sherlock was like "LEAKY PIPES! COINCIDENCE!" and John was like "haha, that means you were wrong about something." Were we supposed to get from that that Sherlock had deliberately dosed him with the gas too, and was still lying about it? But he went and did his chemical analysis of the sugar after John had his episode, so he must have still been thinking it was the sugar then...

ETA: This confused me at the time, because I'd been assuming it was a gas pretty much from the beginning (well 'desperately hoping it wasn't a gas because I wanted it to be more clever than that') so I didn't realize Holmes ever thought it was the sugar until he said it outright.